By definition, a thermal detector is used to measure the amount of incident radiant flux, for example the power of electromagnetic radiation in the infrared ray region.
As far as infrared detectors are concerned, devices designed in the form of an array which are capable of operating at ambient temperature, i.e. which do not require cooling, are known—these are contrasted with detection devices referred to as “quantum” detectors which can only operate at extremely low temperature, typically at the temperature of liquid nitrogen.
These detectors generally comprise a sensing element which can be warmed by infrared radiation which is characteristic of the temperature and emissivity of observed bodies. An increase in the temperature of the sensing element produces a variation in an electrical property of the sensitive material: build-up of electric charges due to the pyroelectric effect, variation in capacity due to change in the electric constant or, more traditionally, variation in the resistance of a semiconductor or metallic material.
In the latter case, one refers to a resistive bolometric detector. The absorbed incident radiation causes a rise in the temperature of the detector which causes variation in electrical resistance. These variations in resistance produce variations in the voltage or current across the terminals of the detector which constitute the signal output by the sensor.
Such an uncooled detector is generally associated with:
means of absorbing the infrared radiation and converting it into heat;
means of thermally isolating the detector so that its temperature can rise due to the effect of the infrared radiation;
thermometric means which, traditionally, use a resistance element;
means of reading electrical signals provided by the thermometric means.
Detectors intended for infrared imaging are produced as a one- or two-dimensional array of elementary detectors on a substrate generally made of silicon which incorporates means of electrically exciting (stimulating) said elementary detectors and means of pre-processing the electrical signals generated by these elementary detectors.
These means of electrical excitation and pre-processing are formed on the substrate and constitute a readout circuit.
In practice, monolithic infrared imaging devices operating at ambient temperature are fabricated by directly connecting an array of sensing elements to a CMOS or CDD type silicon multiplexing circuit.
A device comprising an array of elementary detectors and an associated readout circuit is generally placed in a package and connected, especially electrically, to its external environment using classic techniques (metal wires and pins). The pressure inside such a package is reduced in order to limit thermal losses. The thermal detector can thus be encapsulated in a vacuum or a gas which is a relatively poor conductor of heat in order to obtain improved performance. The package also has a window that is transparent to the radiation to be detected.
In order to observe a scene using this detector, the scene is projected through suitable optics onto the array of elementary detectors and clocked electrical stimuli are applied via the readout circuit (provided for this purpose) to each of the elementary detectors or to each row of such detectors in order to obtain an electrical signal that constitutes an image of the temperature reached by each elementary detector.
This signal is then processed to a greater or lesser extent by the readout circuit and then, if applicable, by an electronic device outside the package in order to generate a thermal image of the observed scene.
The performance of uncooled bolometric detectors is, however, largely dependent on mastering the fabrication and integration of extremely high-performance bolometric materials into very lightweight structures consisting of bolometer micro-bridges that are thermally isolated from the readout circuit in order to exploit the latter to the full in terms of the signal-to-noise ratio.
Detectors such as these which are capable of offering high performance therefore demand, in particular at the level of the sensitive material, good thermal isolation of the active layer from its support as well as a sensitive material which is highly sensitive to the effect used to convert a rise in temperature into an electrical signal. The first two conditions are met by thin-film implementation.
The prior state of the art describes various ways of arranging the various components of the elementary detectors. A classic layout is shown in FIG. 1 in relation to a bolometer.
Schematically, this type of detector is built in the form of a membrane suspended above substrate (1) acting as a support and fixed to the substrate by anchoring points (5) referred to as “posts” which conduct electricity. This membrane comprises a thin film (typically 0.1 to 1 μm) of temperature-sensitive bolometric material (2), two coplanar or parallel electrodes (not shown) and absorber (3).
The term “absorber” denotes one or more layers or arrangements of layers, the function of which is to capture electromagnetic radiation in order to convert it into heat and to transfer its temperature to thin film (2) which acts as a thermometer.
The prior state of the art makes provision for various thermometers (2), including the thermistor which is one widely-used option. In particular, many documents and publications describe various bolometric structures based on a semiconductor material.
The sensitive material can thus be made of slightly or highly resistive p-type or n-type polycrystalline or amorphous silicon. It may also be made of vanadium oxide (VOx) made in a semiconducting phase.
Generally speaking, the sensitive material rests on an insulating support (SiO2, SiO, SiN, etc.) which ensures the bolometric structure is mechanically rigid. It can also be completely encapsulated using one of these insulating materials.
Support substrate (1) typically consists of an integrated electronic circuit on a silicon wafer comprising, firstly, devices for driving and reading the thermometer and, secondly, multiplexing components which male it possible to serialise the signals obtained from the various thermometers and send them to a reduced number of outputs so that they can be analysed by a conventional imaging system. The circuit can be located underneath the detector or be located further away on the substrate. This circuit may also amount to an interconnection network, the function of which is to link the electrical outputs of the detector to an information-processing circuit located elsewhere.
Electrical interconnection between thermometer (2) and the read components located on substrate (1) is ensured by a layer, generally a metallic layer, which is placed on the thermal isolation devices described below.
The sensitivity of the thermal detector is known to be improved by introducing isolating “arms” (8) between support substrate (1) and membrane (3) which are designed to limit the thermal losses of the membrane, thus maintaining the rise in the temperature of the membrane. These flat, elongated and very narrow structures consist of layers which are as thin as possible, they must also be electrically conductive but thermally resistive.
In this type of device, the readout circuit applies, via posts (5) and arms (8) and at least two conductive parts or electrodes (not shown), an electric current which flows through the structure parallel to the plane of the bolometric detector. This current then flows through bolometric material (2), the resistivity of which varies with temperature.
In the embodiment described, the isolation devices are located in the same plane as the layer of biometric material or are produced underneath the latter. When implemented in the plane of the bolometric plate, this system of arms has a deleterious effect in terms of the fill factor of the pixel or elementary detector. Experience shows that it is not possible to significantly increase the length of arms (8) or reduce their width and/or thickness without affecting the rigidity of the structure. In fact, these elements are a mechanical weak point which affects the stability of the micro-bridges which may pivot or deform and therefore come into contact with the substrate, thus adversely affecting thermal isolation and consequently significantly impairing the performance of the detector.
Moreover, one of the layers that constitute these thermal isolation devices is generally an electrically conductive layer, the function of which is to ensure electrical connection between the detector and the readout circuit. Reducing the width and/or thickness of these elements results in increased electrical resistance when accessing the detector and, if this resistance is too high, this adversely affects optimal biasing of the detector.
In addition, the materials used to produce these electrically conductive layers are also good thermal conductors. The presence of this layer in the thermal isolation devices can therefore significantly degrade their thermal isolation and consequently degrade the performance of the detector.
The technical problem which the present invention aims to solve is therefore to propose an alternative arrangement of thermal detectors, especially bolometers, capable of ensuring satisfactory electrical connection while improving the thermal isolation of the bolometric material.